There are many document processing operations wherein an image of each document is required for data processing or for archival purposes. In processing bank checks through the commercial banking system, for example, it is desirable to digitally record images of bank checks for data processing and for archival purposes. Moreover, it is also desirable to verify the quality of the captured image before the check leaves the possession of the image recording entity. In this way, documents which were not properly imaged may be re-recorded before being destroyed or otherwise taken out of possession of the entity requiring the image.
Documents such as bank checks and similar instruments present certain problems in maintaining image legibility due to the presence of hand-written information on the document as well as obliteration of data by background patterns and pictorial printing, contrast and poor quality printing. The long, stroke-like images created by hand-written or hand-printed information, as well as typewritten information, including images of arabic numerals, English letters and signatures, may have varying intensity depending on the writing instrument, the penmanship of the person entering the information on the document or the type of printing apparatus, in the case of machine printed information. Accordingly, analysis of these stroke-like images is deemed a critical image feature to be considered when verifying image quality or legibility.
Another problem associated with capturing legible images of documents such as bank checks pertains to the elements of the image which are created by pictorial backgrounds on the face of the check and the color of the check paper. Moreover, there has heretofore been a tradeoff in setting the threshold sensitivity of document scanning devices because too high a sensitivity results in undesired low contrast features being recorded as part of the image. For example, features such as smudges, background patterns, dropout inks, printing on the other side of the document, or even the paper fibers themselves may be included in the captured image and interfere with reading the desired information. These extraneous features ("noise") also increase the file size for the image and increase the amount of information storage required by digital recording and storage equipment, thereby increasing image transmission time over data networks, for example. Of course, if the scanning device is set to capture an image at a lower sensitivity threshold, this increases the risk of not capturing important hand and typewritten information if such is of lower contrast than might be anticipated. Thus, the optimum normal sensitivity image is one wherein the captured image includes as much of the desired information as possible with little or no extraneous information or "noise".
Although prior efforts have been made to measure image quality by analysis of the entire document or preselected areas of the document, these efforts have been limited to measuring global properties of the image, such as identifying the existence of a totally black or totally white image, which analysis would indicate a failure of the processing equipment. Moreover, the use of histograms reflecting the distribution of the total number of black pixels across a document can also be a quality indicator but the acceptability criteria is based on predetermined averages of pixel distribution. Another technique which has been used is based on counting the total number of small features on a document to indicate the existence of a "high noise level" or by analyzing the compressed image size to indicate a noisy image or an image with excessive pixel dropout. These prior efforts at image analysis are based on predetermined acceptable limits for the global parameters above mentioned and do not consider the localized content of information on a particular document. Accordingly, such techniques do not work well with documents that have a wide variety of image pixel content and have important information in certain specific regions of the image such as, for example, bank checks and similar financial instruments. Another example of the limitations of the above-mentioned global techniques would be in imaging certain types of forms which normally have a great deal of image content even if the form is blank or not filled in with the full amount of information. Accordingly, the global techniques have difficulty in discriminating a blank form from one that has information filled in. Accordingly, the present invention addresses the need to analyze the quality of a captured image of a particular document being scanned and the acceptability of information defects in the image is based on localized analysis of critical elements in the image.
Accordingly, there has been a continuing need to verify image quality and legibility for documents which are being electronically scanned for image capture and storage so that an inferior image may be identified in a timely manner to allow intervention and re-recording of the image while the document is still available for doing so. The present invention contemplates a unique method for verifying image quality and acceptability for electronically-recorded images of the types of documents discussed in detail herein. However, those skilled in the art will appreciate that the method may be used in connection with verification of image quality for other types of documents or for recorded images of other objects.